1. Field of the Invention
The present invention relates to a method for writing servo onto disks of a hard disk drive.
2. Background Information
Hard disk drives contain a plurality of magnetic heads that are coupled to rotating disks. The heads write and read information by magnetizing and sensing the magnetic fields of the disk surfaces. Each head is attached to a flexure arm to create a subassembly commonly referred to as a head gimbal assembly (“HGA”). The HGA's are suspended from an actuator arm. The actuator arm has a voice coil motor that can move the heads across the surfaces of the disks.
Information is typically stored in radial tracks that extend across the surface of each disk. Each track is typically divided into a number of segments or sectors. The voice coil motor and actuator arm can move the heads to different tracks of the disks.
FIG. 1 shows a typical track that has a number of fields associated with each sector. A sector may include an automatic gain control (“AGC”) field 1 that is used to adjust the strength of the read signal, a sync field 2 to establish a timing reference for the circuits of the drive, and ID 3 and Gray Code 4 fields to provide sector and track identification.
Each sector may have also a servo field 5 located adjacent to a data field 6. The servo field 5 contains a plurality of servo bits A, B, C and D that are read and used to position the head 7 relative to the track.
The fields 1–5 must be written onto the disk surfaces during the manufacturing process of the disk drive. These fields are typically written with a servo writer. The servo tracks are sometimes written using a number of spiral servo tracks initially written onto the disks. FIG. 2 shows an example of a number of spiral servo tracks written onto a disk. Using spiral servo tracks is sometimes referred to as an Ammonite servo write process. The spiral servo tracks are used to write the final radial servo tracks that are utilized during the normal operation of the disk drive. This process is described in U.S. Pat. No. 5,668,679 issued to Swearingen et al.
As shown in FIG. 3, the disk has a plurality of spiral servo patterns incrementally spaced across the surface of the disk. To write servo the servo writer reads the disk to detect a spiral servo signal. The servo writer uses the detected spiral servo signal to time the writing of a permanent A, B, C and D servo burst pattern.
As shown in FIG. 4, the reading process to sense the spiral servo signal occurs during a “timing window” that is periodically opened by the servo writer. Timing imperfections or thermal instability may create a non-uniform pattern of spiral servo signals. As shown in FIG. 5, the non-uniformity may cause the timing window to miss the spiral servo signal during the A, B, C and D servo bit writing process. It would be desirable to adjust the timing window to compensate for irregularities in the spiral servo signals.